Those skilled in the field of gas well technology are aware that it is important to determine the permeability of the rock through which a well bore, drilled in search for gas, passes. Even if the well bore intersects a gas bearing stratum, significant levels of gas production can be obtained from the well only if the gas bearing stratum is sufficiently permeable to allow the gas to migrate readily to the well bore.
The permeability of a material is a measure of the rate that a fluid passes through the material under a pressure gradient. Prior art techniques for measuring permeability have been of two types. The first type, known as "plug methods", use a plug or slab of material for testing, establish a pressure gradient across this plug and measure the gas flow through the plug. Such methods suffer from the disadvantage that they require the preparation of a substantial plug of material, and such samples are not routinely available from most well drills, since the drills normally break up the rock being drilled into small drill cuttings or dust. Accordingly, it will normally be necessary to either interrupt the drilling process to produce a suitable core or else to cut cores laterally of the well bore after drilling has been completed; either approach obviously leads to undesirable delay. Plug methods also suffer from the serious disadvantage that the results are sensitive to the effects of fratures induced during sample preparation, and this sensitivity becomes enormous for low permeability samples. For example, measured permeabilities for shale may be as low as one picodarcy. A single 5 micron induced fracture 1 cm. long in a typical plug prepared from such shale will produce a permeability value in error by a factor of 1000. When preparing plugs from friable material such as shale, it is virtually impossible to avoid inducing fractures which will produce serious error in permeability measurements made by the plug technique.
The other major type of prior art permeability measuring methods is the so-called pressure pulse type. In pressure pulse methods, the pressure on one surface of a sample is varied with time, thereby creating internal pressure gradients within the sample, and the flow of fluid into or out of the sample produced by these internal pressure gradients is measured. Pressure pulse methods have the advantage that, since access to only one surface of the sample is required, the sample may be in the form of a slab, drill cuttings or dust (such as that produced by air rotary drills). In addition, pressure pulse methods are much less sensitive to the effects of induced fractures. However, as with plug methods, pressure pulse methods require relatively complicated methods and apparatus, and pressure pulse methods are also relatively slow. In practice, pressure pulse methods can only be conducted in the laboratory and are not suitable for routine use at the drill site.
It is also known to locate fractures in samples by allowing a dye to penetrate the sample, then spraying the sample with a white powder which shows where dye has entered fractures in the sample. Although such a dye penetrant method of locating fractures may give some qualitative indication of the permeability of a sample, it is incapable of being made sufficiently quantitative to be useful in determining permeability of rock strata during drilling of gas wells.
There is thus a need for a method for determining permeability of rock samples which permits permeability measurements to be made on samples in the forms of slabs, drill cuttings or similar small particles which is relatively simple and quick to carry out so that it can be routinely used at the well site to provide permeability measurements while drilling is in progress. This invention seeks to provide such a method.